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Children study in a class located near the brickmaking kilns in Kolkata, India. Workers who know how to read and count will be better able to make sure they are treated fairly by employers.

On the outskirts of India's third-largest city, 5,000 partly blackened chimneys stand 100 feet high, belching smoke into the sky over millions of reddened bricks below. Some of the bricks are stacked neatly into huge square-cornered stacks, and still more, innumerable, are piled roughly – some broken, some chipped and cracked, as if tipped wantonly from a wheelbarrow.

Here around 1.25 million low-caste migrant workers and their dependents spend six months each year dredging clay from nearby lakes or molding bricks under the scorching sun, or lugging back-breaking hods. It is seasonal work, done by India's lowest castes, or in some cases, dirt-poor immigrants from neighboring Bangladesh.

Weather-beaten Ram Dayal, whose home is in Gazpar in Uttar Pradesh, a 24-hour train ride away, says has worked these kilns for 25 years. Asked his age, he laughs and says he doesn't know exactly. “I have a son about your age though,” he tells me.

He's getting ready to go home – “in two or three days” – as the monsoon season approaches, rains that make brick-baking impossible. For now, however, he spends 12 hours a day fueling and stoking a huge kiln, baking around 400,000 bricks at a time. “We are paid 4,700 rupees [$84] a month,” he says.

The soft, wet bricks are stacked in a basketball court-sized area around the stack. With venting space between the wet bricks, they are covered in a layer of stone and baked for two weeks. Keeping the fires lit, Ram Dayal and Murai Rajbal share two, six-hour shifts over each 24-hour period. In between, they mostly sleep in a nearby shack, which, astonishingly, sits right on top of the baking mass of bricks and a short stone's throw from the chimney stack behind.